fix(cursor): cache global DB snapshot and batch bubble lookups on import

[not-so-fat]

Summary

• Cursor composer bubbles live in a multi-GB global state.vscdb; import was copying it once per workspace and querying each bubble individually.
• Cache the global snapshot per process (invalidate on mtime/size) and batch cursorDiskKV lookups with IN queries.
• On a machine with a ~4GB global DB and 31 workspaces, full Cursor import drops from ~259s → ~7–13s with identical message counts.

Diagnosis

Phase	Before	After
Scan all sources	~1.1s	unchanged
Import 31 Cursor workspaces	~259s	~7–13s
Warm-cache workspace import	~6s+ each	21–695ms

Root cause: parseCursorWorkspace() called openSnapshot() on the global DB for every workspace. With a large Cursor history that is mostly disk-copy overhead, plus thousands of per-bubble SQL round-trips.

Test plan

• node test/make-cursor-fixture.mjs + parse fixture (2 sessions, 7 events)
• Profiled real machine: 14,903 events before/after match
• Local dev import wizard: performance confirmed good on reporter's machine
• Sequential import of 4 Cursor projects via /api/import (wizard step 3 path): first 5.3s (global copy), then 826ms / 122ms / 29ms